COMPARISON OF ENDPLATE CONTACT FORCE PROFILES FOR EXPANDABLE VERSUS NONEXPANDABLE LUMBAR INTERBODY CAGES
+1,2Cheng, L.C.; 1,2Currey, J.M.,; 1,2Modak, A.; 1,2McClellan, T.R.; 1,2Pekmezci, M.; 1,2Buckley, J.M.; 1.3Ames, C.P.
+1Biomechanical Testing Facility, UCSF/SFGH Orthopaedic Trauma Institute, San Francisco, CA, +2Department of Orthopaedic Surgery, University of
California at San Francisco, San Francisco, CA 3Department of Neurosurgery, University of California at San Francisco, San Francisco, CA
AmesC@neurosurg.ucsf.edu
INTRODUCTION
Both expandable and non-expandable interbody cage designs are
available to surgeons for cases involving spinal reconstruction and
stabilization following single level lumbar corpectomy [1]. Different
expandable cage designs are on the market [2], but all involve a manual
mechanism to intraoperative adjust the compressive load applied by the
cage against the vertebral body endplates. Non-expandable cages do not
have such a mechanism, and surgeons rely on cutting the cage to the
appropriate length to achieve a “press-fit.” The clinical reasoning behind
the expandable cage design is that the adjustability of the device will
allow the surgeon to achieve the maximum contact area between the
cage and the vertebral endplate without “overstuffing” the interbody
space and thus causing a sagittal-plane deformity. While anecdotal
evidence from previous clinical [3] and biomechanical [4] studies exists
to suggest that the expandable cage provides for a more consistent and
stable interface between the cage and the endplate, as of yet there has
not been a controlled study investigating the differences in the endplate
contact force profile between expandable and non-expandable cage
designs. As such, the goal of this study is to fully characterize the
magnitude and distribution of contact forces along the vertebral endplate
for typical expandable versus non-expandable cage designs (Stryker
Spine).
using this particular device design (VLIF, Stryker) with sufficient data to
safely avoid overloading the endplates.
RESULTS
For normal lordotic angles, the expandable cage design had a
significantly greater contact area than the non-expandable cage design,
(p=0.03,unpaired t-test; Table 1). Contact area did not differ with
lordotic angle for the expandable cage design (p>0.05, repeat-measures
ANOVA); however, the COP location was significantly more anterior
for the applied hyperlordotic endcap than the normal and hypolordotic
cases. There were no differences in total endplate contact force between
expandable vs. non-expandable cage designs nor between different
imposed lordotic angles for the expandable cage design. The necessary
insertion torque to achieve full deployment of the expandable cage was
2.4±0.9 Nm, and these values did not vary with the lordotic angle
(p>0.05, repeat-measures ANOVA).
Table 1: Endplate contact pressure profile data. (*) denotes a
statistically significant difference versus expandable cage design with
normal endplates. The p value is given. No statistically significant
differences were found among endplate force or COP-ML.
Lordotic
Angle
METHODS
Human thoracolumbar spines (N = 6, T12-L2 & L3-L5; 81.5±10.7
y.o. 5 Male, 1 Female) were harvested from fresh-frozen cadavers and
segmented into two spinal sections (T12-L2 and L3-L5). One spinal
section from each donor was assigned to the expandable and nonexpandable cage treatment groups. The superior and inferior vertebrae of
each section were potted in quick-set resin (Smooth Cast 300), and
specimens were mounted to a custom-designed table-top test fixture that
applying a uniform 100 N compressive preload to the specimen to
simulate physiologic intraoperative loads during right lateral decubitus
patient positioning [5] (Figure 1).
Figure 3: Typical endplate contact pressure profiles for a (left)
expandable versus (right) non-expandable interbody cage. Both images
are shown for T12-L2 vertebrae with neutral cage endcaps. Note the
greater contact area with the expandable cage design.
Figure 1:(left) Biomechanical test set-up. Compressive load is applied
via an air bladder, and net force through the spinal section is monitored
with an in-line load cell. Figure 2:(right) Tekscan pressure film inserted
between a non-expandable cage inferior endcap and vertebral endplate.
Spinal reconstructions were performed by trained orthopaedic
surgeons. Corpectomies were performed at L1 or L4, and real-time
tactile pressure measurement film (F-scan Tekscan; #4000 sensor, PSI:
1,500) was positioned along the inferior vertebral endplate (Figure 2).
Either an expandable (VLIFT, Stryker) or non-expandable (VBOSS,
Stryker) cage was inserted into the interbody space and secured against
the endplate. Identically sized interbody cages and cage endcaps were
used for each donor. Supplemental anterior hardware was added to
further secure the cage [6], as would be done clinically. For the nonexpandable cages, the lordotic angle of the cage endcap was not varied,
and a best-fit angle was used in each instance. For the expandable cage
design, hypo-lordotic, hyper-lordotic, and best-fit endcaps were applied
sequentially, and mechanical data was collected for each deployment.
The following outcome measures were recorded for each surgical
case: 1) total endplate contact force on final positioning (pressure film)
and 2) location of the center of pressure between the cage and the
endplate (pressure film). For the expandable cage, the necessary manual
torque to fully deploy the cage was also measured to provide clinicians
DISCUSSION
The results of this study suggest that expandable cage designs
provide greater contact area between the cage endcaps and the vertebral
endplate, potentially providing a more stable base for bony union. For at
least the expandable cage design, the distribution of load along the
endplate is sensitive to imposed non-normal lordotic angulation, with the
insertion of hyperlordotic endcaps causing a substantial anterior shift of
the center of pressure on the endplate. This shift may increase the
likelihood of vertebral body failure under repeat loading conditions. Our
results suggest that torque feedback on distraction may not be a reliable
indicator of cage fit since torque values did not vary with drastic
changes in the deployed lordotic cage angle. Future work will focus on
expanding the sample size of this study to further elucidate differences
in deployment characteristics between expandable vs. non-expandable
cage designs.
ACKNOWLEDGEMENTS
Funding and hardware provided by Stryker Spine. Thanks to M. Fong.
REFERENCES
[1] Kandziora F. et al., Spine, 2004. [2] Khodadadyan-Klostermann C et
al, Chirurg, 2004. [3] Ernstberger T et al., AOTS, 2005. [4] Reinhold M,
et al., Unfallchirurg, 2007. [5] Gilsanz V et al, Radiology, 1994. [6]
Payer M et al., Acta Neurochir (Wien), 2006.
Poster No. 1532 • 56th Annual Meeting of the Orthopaedic Research Society